Further work is planned on a continuing project to identify the regulatory mechanisms involved in maintaining membrane physical properties in an optimal state for metabolic function. In this budget period, emphasis will be placed on studying the factors which control the activity of enzymes for lipid metabolism, such as fatty acid desaturases. Large fluctuations in desaturase activity have been noted when the model organism, Tetrahymena pyriformis, is shifted from one growth temperature to another. These fluctuations are more closely correlated to membrane fluidity changes than to changes in temperature per se. We will expand our approach to include additional physical chemical work, particularly fluorescence polarization and electron spin resonance techniques. These methods as well as continued lipid analyses and isotope incorporation studies will be applied not only to the response and acclimation of cells to temperature change but also to changes in the concentration of cations, including Na ion, K ion, Mg ions and Ca ions, and changes in pressure. Additional experiments will characterize the effects of membranes of selected agents such as dimethylsulfoxide and the ionophores X-537A and A-23187. BIBLIOGRAPHIC REFERENCES: Fukushima, H., C. E. Martin, H. Iida, Y. Kitajima, G. A. Thompson, and Y. Nozawa, Biochimica et Biophysica Acta, 1976, in press. Changes in Membrane Lipid Composition during Temperature Adaptation by a Thermotolerant Strain of Tetrahymena pyriformis. Skriver, L., and G. A. Thompson, Jr., Biochimica et Biophysica Acta, 1976, in press. Environmental Effects on Tetrahymena Membranes. Temperature-induced Changes in Membrane Fatty Acid Unsaturation are Independent of the Molecular Oxygen Concentration.